SSM2120 Datasheet, PDF (3/12 Pages) Analog Devices – Dynamic Range Processors/Dual VCA

+V_C1
SSM2122
INPUT 1
OUTPUT 1
REC_{IN 1}
2V
–V_C1 CFT 1
+V_C2
INPUT 2
OUTPUT 2
REC_{IN 2}
2V
–V_C2 CFT 2
SSM2120/SSM2122
FULL
WAVE
RECTIFIER
|I_IN|
V+ THRESH 1
CON_{OUT 1}
LOG AV 1
V–
FULL
WAVE
RECTIFIER
|I_IN|
V+ THRESH 2
CON_{OUT 2}
LOG AV 2
V–
Figure 1. SSM2120 Block Diagram
VOLTAGE-CONTROLLED AMPLIFIERS
The two voltage-controlled amplifiers are full Class A current
in/current out devices with complementary dB/V gain control
ports. The control sensitivities are +6 mV/dB and –6 mV/dB. A
resistor divider (attenuator) is used to adapt the sensitivity of an
external control voltage to the range of the control port. It is
best to use 200 Ω or less for the attenuator resistor to ground.
VCA PERFORMANCE
Figures 2a and 2b show the typical THD and noise performance
of the VCAs over ±20 dB gain/attenuation. Full Class A operation
provides very low THD.
0.03
VCA INPUTS
The signal inputs behave as virtual grounds. The input current
0.01
compliance range is determined by the current into the reference
current pin.
REFERENCE PIN
The reference current determines the input and output current
compliance range of the VCAs. The current into the reference
pin is set by connecting a resistor to V+. The voltage at the
reference pin is about two volts above V– and the current will be
I _REF
=
[(V +) – ((V – ) + 2 V
R_REF
)]
The current consumption of the VCAs will be directly pro-
portional to I_REF which is nominally 200 µA. The device will
operate at lower current levels which will reduce the effective
dynamic range of the VCAs. With a 200 µA reference current,
the input and output clip points will be ± 400 µA. In general:
I_CLIP = ± 2 I_REF
VCA OUTPUTS
The VCA outputs are designed to interface directly with the virtual
ground inputs of external operational amplifiers configured as
current-to-voltage converters. The outputs must operate at virtual
ground because of the output stage’s finite output impedance.
The power supplies and selected compliance range determines
the values of input and output resistors needed. As an example,
with ± 15 V supplies and ±400 µA maximum input and output
current, choose R_IN = R_OUT = 36 kΩ for an output compliance
range of ± 14.4 V. Note that the signal path through the VCA
including the output current-to-voltage converter is noninverting.
0.003
–20
–10
0
10
20
GAIN – dB
a. VCA THD Performance vs. Gain
(+10 dBV In/Out @ 1 kHz)
–70
–80
–90
–20
–10
0
10
20
GAIN – dB
b. VCA Noise vs. Gain (20 kHz Bandwidth)
Figure 2. Typical THD and Noise Performance
REV. C
–3–